This invention relates generally to a dispensing pump, and more particularly to such a pump of the pressure build-up variety in which communication between the liquid inlet and the pump chamber is established and interrupted by a valve controlled flow passage located in a small diameter piston member movable with a valve member which opens and closes the discharge passage.
Pressure build-up pumps of the general type hereof are disclosed in U.S. Pat. No. Re. 28,366 and in U.S. Pat. No. 4,051,983, both commonly owned herewith, as well as in U.S. Pat. No. 4,025,046 and in German Offenlegungsschrift No. 27 13 447. All of these pumps function in a similar manner in that a valve seat, carried by a main piston, is opened upon the build-up of pressure in the pump chamber and is closed when that pressure is overcome by the force of a spring moving a valve member into its closing position. When the discharge passage is open, the fluid inlet to the pump chamber is interrupted and, while the discharge passage is being closed, the fluid inlet to the pump chamber is re-established.
The inlet to the pump chamber, in the Reissue Patent as well as in the '983 patent, is controlled by a ball-type check valve associated with the smaller diameter piston so as to be unseated during the upstroke of the pistons during a differential pressure on opposite sides of the lower or smaller diameter piston. In another embodiment set forth in the Reissue Patent, the smaller diameter piston has a flexible wall which is caused to move away from the wall of the lower cylinder bore in response to such pressure differential.
In both the '046 patent and in the German Offenlegungsschrift, means movable during operation of the piston engages an element extending into the lower bore of the pump cylinder for interrupting communication between the inlet and the pump chamber. In the '046 patent, such a means comprises a sleeve made rigid with either the piston member or with the valve member, and in the German Offenlegungsschrift such a means comprises a rod or sleeve extension of the valve member.